1. Field of the Invention
The present invention relates to a disc player, and more particularly to an optical pick-up apparatus for a disc player which can be manufactured in a small size.
2. Prior Arts
An optical pick-up is an apparatus for recording information on an optical disc or reading the recorded information in the optical disc by using a laser beam. The optical pick-up is installed below the optical disc and linearly moves from a center of the optical disc to an outer periphery of the optical disc so as to detect a desired track position on the optical disc. By use of the optical pick-up, a beam radiated from a holographic element is introduced into a pit formed on a recording surface of the optical disc. When the beam is reflected from the optical disc, the optical pick-up detects a digital signal from the reflected beam through an optical system. The detected digital signal is converted to an analog signal by a D/A converter so that information recorded in the optical disc is reproduced.
Generally, the optical pick-up includes a light source for producing the laser beam, a beam splitter for directing the laser beam emitted from the light source toward an objective lens which focuses the laser beam on the surface of an optical disc, and a photo diode for receiving the laser beam reflected from the surface of the optical disc.
Information tracks, on which information is recorded, are concentrically or spirally formed on the recording surface of the optical disc, and the optical pick-up emits the laser beam onto the information tracks so as to record data on a predetermined position of the optical disc or to reproduce the data from the predetermined position.
In order to accurately record or reproduce the data, a focusing error control and a tracking error control should be performed. That is, the laser beam emitted onto the information track from the optical pick-up should trace the information track while the disc player is being operated. In addition, a distance between the recording surface of the optical disc and the object lens should be constantly maintained. Thus, a disc recording/reproducing apparatus detects a servo error signal including a focusing error signal and a tracking error signal by utilizing the beam reflected from the information track, and then applies electric signals to both a focusing circuit and a tracking circuit based upon the detected servo error signal, thereby moving the optical pick-up in the focusing and tracking directions.
FIG. 5 shows such a conventional optical pick-up 10. As shown in FIG. 5, conventional optical pick-up 10 includes a yoke plate 44, to which suspension support plates 46 are attached, and an objective lens holder 14 which is suspended above yoke plate 44 by means of suspensions 50 and supports an objective lens 12.
A laser beam through hole 45 is formed at the center of yoke plate 44 to allow the laser beam to pass therethrough, and a pair of inner yokes 38 and 40 and a pair of outer yokes 36 and 42 are arranged on yoke plate in opposition to each other. Inner yokes 38 and 40 are respectively surrounded by focusing coils 24 and 26. Permanent magnets 32 and 34 are respectively arranged between inner yoke 38 and outer yoke 36, and between inner yoke 40 and outer yoke 42. Also, tracking coils 28 and 30 are respectively arranged between focusing coil 24 and permanent magnet 32, and between focusing coil 26 and permanent magnet 34. Suspension support plates 46 are attached to one end of yoke plate 44, and an end plate 48 is attached to the rear surface of suspension support plates 46.
Suspensions 50 are manufactured by conductive materials, and receives an electric current from a current source (not shown) through end plate 48. One end of each suspension 50 is fixed to end plate 48 by passing through suspension support plates 46, and other end of each suspension 50 is fixed to support pieces 16 and 18 which are integrally formed with both sides of object lens holder 14.
Optical pick-up 10 constructed as described above operates as follows.
Firstly, when an optical disc, such as a CD, has been loaded in a cartridge of the disc player, optical pick-up 10 moves to a disc loading position by means of a driving device (not shown). Next, a laser beam is emitted toward a recording surface of the optical disc from a laser beam source (not shown) through object lens 12. Then, the laser beam is reflected from the recording surface and inputted into a photo detector (not shown).
Upon receiving the reflected laser beam, the photo detector detects a focusing error caused by vertical motion of the optical disc and a tracking error caused by a radial deviation of the optical disc and sends focusing and tracking error signals to a microcomputer (not shown). Then, in order to correct the focusing and tracking errors, the microcomputer sends an electric signal to both a focusing circuit and a tracking circuit so that an electric current is applied to both focusing coils 24 and 26 and tracking coils 28 and 30. When the electric current is applied to focusing coils 24 and 26, object lens holder 14 moves in a focusing direction F by means of an electromagnetic-interaction generated between permanent magnets 32 and 34 and focusing coils 24 and 26, thereby correcting the focusing error.
In addition, when the electric current is applied to tracking coils 28 and 30, object lens holder 14 moves in a tracking direction F by an electromagnetic-interaction between permanent magnets 32 and 34 and tracking coils 28 and 30, thereby correcting the tracking error.
However, in order to accurately move lens holder 14 in the focusing or tracking direction, the tracking and focusing coils should be precisely positioned at their operating positions with respect to permanent magnets 32 and 34. Though focusing coils 24 and 26 can be precisely wound around lens holder 14 with respect to permanent magnets 32 and 34, it is difficult to precisely install tracking coils 28 and 30 at their operating position with respect to permanent magnets 32 and 34.
For this reason, the manufacturer should correct the positions of tracking coils 28 and 30 so as to obtain precise positions with respect to permanent magnets 32 and 34 under the condition that tracking coils 24 and 26 are temporarily fixed to focusing coils 24 and 26. Accordingly, assembling tracking coils 28 and 30 into focusing coils 24 and 26 is very complicated, so the productivity of pick-up 10 is decreased.
In addition, when the electric current is applied thereto, tracking coils 28 and 30 adhered to focusing coils 24 and 26 make electromagnetic-interaction in relation to permanent magnets 32 and 34 so as to move lens holder 14 in the tracking direction. However, since the side surfaces of tracking coils 28 and 30, which face each other, can make the electromagnetic-interaction only with respect to permanent magnets 32 and 34, the operating efficiency thereof is reduced.
Furthermore, the pair of outer yokes 36 and 42 for fixing permanent magnets 32 and 34 thereto, the pair of inner yokes 38 and 40 for forming a magnetic closed circuit, and suspension support plate 46 for fixing end plate 48 thereto are provided in yoke plate 44 by bending work. This kind of bending work needs to be carried out accurately, so the working efficiency is decreased.
In addition, the conventional optical pick-up apparatus requires the pair of inner yokes 38 and 40, the pair of outer yokes 36 and 42, the pair of focusing coils 24 and 26, two pairs of tracking coils 28 and 30, and the pair of permanent magnets 32 and 34, so yoke plate 44 on which the above elements are installed should be designed in a relatively large size. Accordingly, not only are many assembling steps required, but also it is difficult to make the optical pick-up apparatus in a small size.